First you need to use qemu-img to resize the img file:
Usage is: qemu-img resize filename size
It helps to temporarily move the wheezy image into D:\Qemu
3.7G seems to be the max you can resize to.
Open comandline and navigate to D:\Qemu
Example: qemu-img resize 2012-09-18-wheezy-raspbian.img 3.7G
Then create a .bat file under D:\Qemu, containing the following launch string: